Sigma’s range of DC lenses are designed for DSLRs with smaller 'cropped-frame' sensors, such as Pentax's APS-C models. DC lens optics are only corrected for use on these bodies and are not compatible with full-frame models.

Like all lenses, you’ll still need to take your DSLR’s field reduction factor into account. Since DC lenses are only designed for use on APS-C bodies, the reduction factor will be 1.5 times for every Pentax APS-C model. So the general-purpose DC 18-50mm lens will deliver a field of view equivalent to a 27-75mm.